help me with the similarity and congruence

A cup is in the form of an inverted square pyramid. When the depth of the water is 10cm, the volume of the water is 187.5cm³. When
327cm³ of water is pored into the the cup, water level rises to 14cm.Find the increase in area of the water surface in the cup.Ans is 18cm²

A cup is in the form of an inverted square pyramid. When the depth of the water is 10cm, the volume of the water is 187.5cm³. When
327cm³ of water is pored into the the cup, water level rises to 14cm.Find the increase in area of the water surface in the cup.Ans is 18cm²

where L is the length of a side of the base and h is the height. So from the first pieces of information you can find L.

Now you add some volume. You know that the result is still going to be a square pyramid, so
where . You can find the new L'. Thus